Currently, as a bonding wire for a semiconductor device connecting between electrodes on a semiconductor device and outer leads (hereinafter, may be referred to as a “bonding wire”), thin wires with a wire diameter of about 15 to 50 μm are mainly being used. A method for bonding the bonding wire is generally a thermal compressive bonding technique with the aid of ultrasound, which uses a general-purpose bonder, a capillary tool used for bonding by passing the bonding wire therethrough, and the like. A bonding process of the bonding wire includes heating and melting a wire tip with arc heat input, forming a ball (FAB: free air ball) through surface tension, performs crimp bonding (hereinafter, may be referred to as “ball bonding”) of this ball onto the electrode of the semiconductor device heated within the range of 150 to 300° C., forming a loop, and finally performs crimp bonding (hereinafter, may be referred to as “wedge bonding”) of a wire onto an electrode of the outer lead. As bonding counterparts of the bonding wire, an electrode structure in which an alloy mainly containing Al is formed as a film on a Si substrate is used for the electrode on the semiconductor device, whereas an electrode structure plated with Ag or Pd is used for the electrode of the outer lead.
Au, which has been mainly used as a material of the bonding wire, has been being replaced with Cu mainly for LSI use. On the background of recent proliferation of electric vehicles and hybrid vehicles, needs for replacing Au with Cu are increasing also in on-vehicle device use.
A Cu bonding wire using high-purity Cu (purity: 99.99% by mass or more) is developed (Patent Literature 1, for example). Cu has the drawback of being more susceptible to oxidation than Au and has problems in that bonding reliability, ball formability, and wedge bondability are inferior. As a method for preventing surface oxidation of the Cu bonding wire, a structure that coats a surface of a Cu core material with a metal such as Au, Ag, Pt, Pd, Ni, Co, Cr, or Ti is developed (Patent Literature 2). A structure that coats the surface of the Cu core material with Pd and coats a surface thereof with Au, Ag, Cu, or an alloy thereof is developed (Patent Literature 3).